This invention pertains to power electronics, electrical power conversion and switch mode power supplies.
Semiconductor half-bridge power circuit topologies are the fundamental building blocks used in most high power converters. Two half-bridge circuits are used to configure a full-bridge topology and three half-bridge circuits are used to configure a three-phase bridge topology. The semiconductor switches used are typically insulated gate bipolar transistor (IGBT) and field effect transistor (FET) devices.
When a semiconductor power switch is switched off, a mono-polar gate drive places the gate of the semiconductor device at the same voltage potential as the source for FET devices or the emitter for IGBT devices. A bipolar gate drive places the gate of the semiconductor device at a voltage potential that is negative with respect to the source or emitter.
Not all applications require bipolar drive voltages. However, at gate transitions, if the parasitic inductance from the gate driver circuit to the semiconductor die is large enough, the gate voltage will ring or oscillate at the characteristic frequency of the tank circuit created by this parasitic inductance. If the peak excursions of this ringing are greater than the turn-on threshold of that device, the semiconductor switch will oscillate on and off for a number of unintended high frequency cycles. This extra switching manifests in higher switching losses and higher levels of radio frequency emissions. When a bi-polar drive is used, the negative turn-off potential moves the peak excursion of the gate oscillation away from the device turn-on threshold by an amount equal to the negative gate drive voltage. If the negative gate potential is large enough with respect to the amount of parasitic inductance and the speed of the rise and fall times of the gate voltage, the peak excursion of the oscillation will not exceed the device turn-on threshold. Typically, high frequency, switch-mode applications employing a number of paralleled semiconductor dies or devices will require bipolar drives.
In a typical half-bridge topology using IGBT devices, the emitter of the bottom switch, the zero voltage reference point for the IGBT drive circuit, is connected to a stationary −DC bus with respect to earth ground. The voltage at bottom switch collector and the top switch emitter move between the +DC bus voltage and the −DC bus and at very high rates of voltage change with respect to time. In the prior art, bipolar power supply voltages for the non-stationary top switch were typically transformer isolated. In a typical prior art, three-phase DC to AC power converter, four transformer isolated power supplies were required; one for all three half-bridge bottom switch drives and one for each (three) top switch drives. This invention not only reduces the number of bipolar drive supplies required from four to one but also enables an integrated circuit, silicon-only solution for providing top switch bipolar drive supplies.
In this patent disclosure, a preferred embodiment for the invention will be disclosed. Therein, a known but enhanced method of providing the top switch positive gate drive power is disclosed as well as a new and novel method of providing the top switch negative gate drive power. The half-bridge topology, and regulation and control thereof, used to illustrate the use of the invention are known.
FIG. 1 illustrates one common, prior art method of providing positive and negative drive voltage potentials to the top or high-side semiconductor switch in a half-bridge topology. AC power source 71 is typically 120 volts, 60 Hertz. AC power source 71 may also be a high frequency AC source to reduce the size of transformer 72. Transformer 72 provides the electrical isolation to enable the secondary windings, common to the emitter of IGBT 20, to operate at a common mode voltage different from AC source 71. Rectifiers 73 and 74 steer current into capacitor 22 and rectifiers 75 and 76 steer current into capacitor 23 in a typical center-tapped bridge arrangement.
FIG. 2 illustrates another common, prior art method of providing positive and negative drive voltage potentials to the top or high-side semiconductor switch in a half-bridge topology. The positive voltage for driver 25 and subsequently for IGBT 20, is provided by a known “bootstrap” circuit arrangement. When IGBT 10 is conducting, current is sourced from power supply 12, flows through bootstrap diode 30, into capacitor 22 and returns through IGBT 10. DC to DC converter 77 converts a portion of the energy stored in capacitor 22 into the negative voltage used by driver 25. This negative charge, with respect to the emitter of IGBT 20, is stored in capacitor 23. The topology of DC to DC converter 77 can vary. Common arrangements include flyback converters, charge pumps and transformer isolated topologies.